This subject comes up so often that it seems worthwhile to make it a permanent library topic: When the holding tank fills with water while they're away from the boat, the first place most people look is the toilet intake. But let's apply a little logic: if the toilet intake were the source of the the water that's filling your holding tank, isn't it logical to assume that water would rise the toilet bowl first? In fact, unless your joker valve (the cup-shaped valved in the toilet discharge) is so worn that it's wide open, water coming through the toilet would seep out so slowly that it would overflow the bowl before it could even begin to fill your holding tank. Therefore, the water filling your holding tank has to be coming into it from another source, and the most obvious source is the tank's overboard discharge thru-hull. Water always tries to sees its own level...and if there's an open hole in the boat, water outside the boat will also try to seek its own level INside the boat. So if a seacock is left open, water will rise in that hose to the waterline on the boat. If there's a vented loop in the line at least a foot above the waterline, water will only rise in the hose to the waterline...it won't climb any higher. But without a loop, it will fill a tank or the toilet or anything else it can that's below the waterline. And if it's the tank's overboard discharge thru-hul,l the macerator won't stop it. If your tank is only partly below the waterline, it will only rise to the waterline...but if your tank is entirely below the waterline, it will fill and then back up into your your toilet and overflow it. The same is true of toilet intakes--if the seacock is left open, and there's no loop above the waterline in the intake line, water will rise in the toilet to the boat's waterline. If the top of the bowl is above the waterline, it will stop rising at the waterline...but if the top of the bowl is at or below the the waterline, it will overflow the bowl. This has sunk more than one boat in its slip--which is why ALL seacocks should be kept closed except when actually in use. "But-but-but" you cry, " it's a PITA to have to open a close seacocks all the time...and besides, there are valves in the toilets that are supposed to prevent this!" Valves FAIL! Keep the seacocks closed! Especially the toilet and holding tank seacocks. Even a vented loop will not stop an effect known as "ram water" that occurs when you're underway: water pushed up a hose by the pressure of the hull against it. Because you're aboard to notice the mess, "ram water" rarely sinks a boat...seawater from the head intake is enough of a mess, but if it's your holding tank that's overflowed back into the toilet causing the bowl to overflow, that's a mess you don't want to have to clean up! Keep the seacocks closed except when actually in use!